Interactions between the actin cytoskeleton and plasma membrane are believed to be responsible for the locomotion of cultured cells, morphogenetic cell movements and metastasis in vivo, and amoeboid chemotaxis. Numerous studies have demonstrated that the cell cortex is a microfilament rich zone with gel-like consistency. It is generally believed that growth, reorganization, sliding, disassembly and membrane attachment of microfilaments in this region are responsible for regulating cell shape and locomotion. Understanding of the way in which these microfilament based reactions are regulated and coordinated represents one of the major unsolved areas of cell biology. We are interested in the mechanisms by which ligand binding to its receptor at the cell surface regulates cell polarity and surface motility resulting in chemotaxis. The organism we have chosen to study is Dictyostelium discoideum the amoeboid stage of which exhibits chemotaxis toward cAMP or folate at different stages of development. We propose to identify the actin binding proteins which are responsible for the regulation of assembly of actin cytoskeleton in the cell cortex, characterized their structure and function in vitro and identify signals which are generated by ligand stimulation that regulate the functions of these proteins.